[R-sig-Geo] Counting overlapping polygons in a given area

[Andrew Bernj]

Dear Roger,

Thank you very much for this example. It was exactly what I needed to
get started on this issue. Especially the final part, using the
development version of rgeos. I have been trying some different things
before and I suspect one of the problems had to do with the fact that
I have just too many cities*religions. It helped a lot with other
problems I had to solve.

Also, I do apologize for not sending my previous code or a toy
exmaple. I will remember that in the future.

Best,

Andrew



2014-04-17 5:19 GMT-04:00 Roger Bivand <Roger.Bivand at nhh.no>:
> On Thu, 17 Apr 2014, Andrew Bernj wrote:
>
>> Hi,
>>
>> I am used to work with ArcGIS but I am new to R spatial functionalities.
>> So, I have been searching in the archives of this list, but did not find a
>> previous question exaclty on what I have to accomplish.
>
>
> On lists in this community, it is usual to follow the instructions, in
> particular to provide a toy example illustrating your problem. If you don't,
> nobody knows whether the suggested resolution matches your problem. You also
> posted HTML-mail; please use plain text only, so that included code may be
> pasted straight into an R console.
>
>
>>
>> It is the following. I have two shapefiles, one with US city and town
>> divisions, where each polygon is a city/town (UScities.shp) and another
>> with some US religious groups, where polygons represent the area where
>> there are significant followers of a given religion (USrelig.shp).
>>
>> So, in UScities.shp of course the polygons do not overlap. While in
>> USrelig.shp polygons do overlap, since followers from different religions
>> can be present in the same place.
>
>
> library(sp)
> library(rgdal)
> dsn <- system.file("vectors", package = "rgdal")[1]
> scot_BNG <- readOGR(dsn=dsn, layer="scot_BNG")
> plot(scot_BNG, axes=TRUE)
>
> gives something like UScities.shp.
>
> set.seed(1)
> pts <- spsample(scot_BNG, n=200, type="random")
> plot(pts, add=TRUE, col="red")
> library(rgeos)
> polys <- gBuffer(pts, width=50000, byid=TRUE)
> plot(polys, add=TRUE, border="red")
>
> gives something like USrelig.shp.
>
> library(rgeos)
> gO <- gOverlaps(scot_BNG, polys, byid=c(TRUE, TRUE))
> dim(gO)
>
> makes a logical matrix with TRUE where scot_BNG[i,] overlaps with polys[j].
>
> count_by_city <- apply(gO, 2, sum)
>
> The output vector is named using row.names(scot_BNG), so if you need
> recognizable names in the output, change the input row.names to something
> informative.
>
> This is OK if the number of cities times the number of religions is
> moderate. If the product is large, try the development version of rgeos
> from:
>
> https://r-forge.r-project.org/R/?group_id=602
>
> and use:
>
> gO <- gOverlaps(scot_BNG, polys, byid=c(TRUE, TRUE), returnDense=FALSE)
> length(gO)
> count_by_city1 <- sapply(gO, length)
>
> and in this case we see that they are equivalent:
>
> all.equal(count_by_city, count_by_city1)
>
> Hope this helps,
>
> Roger
>
>>
>> What I want do is to count how many religious groups according to
>> USrelig.shp there are in each city present in UScities.shp. It means, to
>> count how many polygons overlap in USrelig.shp for each polygon division
>> given by UScities.shp.
>>
>>
>> Is there a way to do that in R? Thanks in advance for any tip you may have
>> on this task.
>>
>> Bernj
>>
>>         [[alternative HTML version deleted]]
>>
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>>
>
> --
> Roger Bivand
> Department of Economics, Norwegian School of Economics,
> Helleveien 30, N-5045 Bergen, Norway.
> voice: +47 55 95 93 55; fax +47 55 95 91 00
> e-mail: Roger.Bivand at nhh.no
>